Multilayer printed circuit boards have been known for some time, but the art has developed a variety of problems. See, for example, U.S. Pat. Nos. 3,934,335 and 4,211,603 and the problems discussed therein which include delamination, layer misalignment, smearing of dielectric material onto circuitry layers and non-uniformly-sized and ill-defined vias, or holes through dielectric layers, for interconnection between layers of circuitry, as well as an inability to produce vias less than about 15 mils in diameter. There have been attempts to solve such problems, the attempts resulting in the development of some novel solutions, but the solutions themselves have often resulted in processes which are complex, time-consuming and relatively expensive. For example, in U.S. Pat. No. 3,934,335 there is disclosed a complex method of fabricating multilayer printed circuit boards involving forming conductive patterns by electrolessly plating a conductive metal onto a layer of dielectric which has first had its surface pitted by treatment with a caustic solution, then sensitized by treatment with successive solutions of tin and palladium salts. As another example, polymer thick film (PTF) technology is used for fabrication of a two conductive layer LED display device in U.S. Pat. No. 4,305,204, which discloses a time-consuming printing process for a double-printed dielectric layer over an etched conductor pattern, the dielectric comprising an epoxy resin and requiring a partial heat cure after the first printing and a full heat cure after the second printing; thereafter, a conductive PTF layer is silkscreened over the double-printed laminated dielectric layer and all three layers are further heat cured.
Thus, there exists a need for a multilayer printed circuit board fabrication process which is simple, fast, inexpensive and is capable of producing boards with sharply defined and uniformly sized vias substantially smaller than 15 mils in diameter and which will resist delamination and short circuiting under operating conditions. This need and other advantages are accomplished by the present invention, which is summarized and described in detail below.